4-Acetonyl-3-nitro-1-phenylmethylpiperidine (Tetrahedron Lett., 31, 3039 (1990)) and 3-nitro-2-phenylpiperidine (Bioorg. Med. Chem. Lett., 5, 1271 (1995) are known, but their pharmacological activities are unknown.
An object of the present invention is to provide piperidine derivatives or pharmaceutically acceptable salts thereof which are useful as pharmaceuticals such as an antitumor agent.